api/metadata: Fix issues detected with hypothesis
Running metadata tests with multiple hyothesis examples uncovered those
issues in api-1-raw-extrinsic-metadata-swhid Web API view:
- RawExtrinsincMetaData only targetting core SWHIds must be provided as test inputs.
- link-next URL for pagination was invalid.
- next_page_token must be encoded before providing it to urlsafe_b64encode.